In recent years, sol-gel chemistry has attracted considerable interest in chemical analysis, in part, to the ease of which inorganic-organic hybrid materials can be fabricated. Silica-based sol-gel materials have received the most attention. Silica-based coatings and monoliths have been synthesized via sol-gel processes and silica-based sol-gel stationary phases represent a rapidly growing area within separation science. The chemical properties of sol-gel materials can be fine-tuned via simple manipulation of ingredients and reactions that commonly take place at ambient temperatures. However, silica-based sol-gel materials also have many drawbacks, including their instability under extreme pHs and harsh solvents.
Other metal oxides, such as aluminum, titania, zirconia and hafnia, have been incorporated into silica-based systems; however, all of these hybrid materials have significant drawbacks. While titania and zirconia both exhibit enhanced mechanical strength and higher stability in extreme pHs in comparison to silica, they are both adsorptive and their surface chemistry differ significantly from that of silica (Winkler & Marme, J Chromatogr. A. 2000, 888, 51; Jiang & Zuo, Anal. Chem. 2001, 73, 686; Tani, & Suzuki, J. Chromatogr. A. 1996, 722, 129; Tsai, et al. J. Chromatogr. B. 1994, 657, 285; Fujimoto, Electrophoresis. 2002, 23, 2929).
Sol-gel titania-PDMS (Kim, et al. J. Chromatogr. A. 2004, 1047, 165) and sol-gel zirconia-PDMDPS (Alhooshani, et al. J. Chromatogr. A. 2005, 1062, 1) coatings fabricated by the present inventors showed stability under highly acidic and alkaline environments. These coatings also provided excellent solvent stability due to their ability to form direct chemical bonding with capillary inner walls. However, the use of titania and zirconia alkoxides as precursors of the silica-based sol-gel material also have significant disadvantages. For example, sol-gel titania/zirconia-silica materials have very rapid reaction rates, leading to instantaneous precipitation of zirconia when water is added to the system (Chang, et al. J. Membr. Sci. 1994, 91, 27). In addition, metal-bound hydroxyl groups on these zirconia and titania-based materials serve as adsorptive sites to polar solutes. This can cause problems such as sample loss, difficulty in reproducibility, sample carryover, peak distortion, and peak tailing (Alhooshani, et al. J. Chromatogr. A. 2005, 1062, 1). Narrow-bore zirconia monolithic columns were prepared by coating the silica surface with a layer of zirconia, followed by washing with dry ethanol and then water to promote hydrolization of zirconium ethoxide. Zirconia monolithic columns were also prepared by a sol-gel process with a solution of acetic acid catalyst, PEG and n-butanol (Randon, et J. Chromatogr. A. 2006, 1109, 19).
Hafnia lies within the same periodic group as titania and zirconia and has a high dielectric constant and good thermodynamic interfacial compatibility with silica. Hoth et al. (Hoth, et al. J. Chromatogr. A. 2005, 1079, 392-396) prepared metal oxide monoliths composed of ZrO2 and HfO2 via in situ synthesis. However, its disadvantages include problems of phase separation during annealing, especially at higher Hf concentrations. In SiO2 sol gels, heat treatment leads to further condensation, thereby strengthening the silica network. The incorporation of Hf in silica networks leads to non-bonding oxygen, which suggests that HfO2 phase separation occurs as temperature increases (O'Dell, et al. Solid State Nuclear Magnetic Resonance. 2008, 33, 16).
Alumina has also been incorporated into silica systems as it has many attractive qualities including excellent pH, thermal and mechanical stability. It also has a high surface area and is capable of ligand exchange, thereby capable of extracting polar compounds. Since alumina has Lewis acidity and basicity as well as a low concentration of Brønsted acid sites (Nawrocki, et al. J. Chromatogr. A. 2004, 1028, 1; Nawrocki, et al. J. Chromatogr. A. 2004, 1028, 31; Claessens & Van Sraten, J. Chromatogr. A. 2004, 1060, 23; Grün, et al. J. Chromatogr. A. 1996, 740, 1), it can undergo both ion and ligand exchange (Nawrocki, et al. J. Chromatogr. A. 2004, 1028, 1). Like zirconia, titania and hafnia, alumina also undergoes very fast reactions in the presence of water and has many adsorptive sites within alumina hydroxyl-terminated polydimethysiloxante (PDMS) coatings, which need to be deactivated (Liu, et al. J. Chromatogr. A. 2006, 1108, 149-157). Fujita et al. (Fujita, et al. J. Non-Cryst. Solids. 2008, 354, 659-664) developed Cr3+-doped macroporous Al2O3 monoliths using the sol-gel method followed by phase separation. This new metal-salt derived method developed by Gash and coworkers enables the synthesis of metal oxide aerogels and xerogels from the corresponding metal salts (Gash, et al. Chem. Mater. 2001, 13, 999; Gash, et al. Chem. Mater. 2003, 15, 3268; Gash, et al. J. Non-Cryst. Solids 2001, 285, 22; Baumann, et al. Chem. Mater. 2005, 17, 395).
In the periodic table, germanium is a metalloid that lies within the same group as silicon, indicating that they share similarities in chemical structures and properties. Germania (Ge—O2) has been shown to be an isostructural analog of silica (Si—O2) (Fang, et al. Anal. Chem. 2007, 79, 9441-9451). Currently, germania is used in the optical fields for making waveguide films with controllable refractive index, photonic devices such as high-density optical data reading and storage devices, upconversion lasers, infrared laser viewers, and indicators (Chen, et al. J. Non-Cryst. Solids. 1994, 178, 135; Brusatin, et al. J. Am. Ceram. Soc. 1997, 80, 3139; Brede, et al. Appl. Phy. Lett. 2000, 63, 729; McFarlane, J. Opt. Soc. Am. B. (1993) 11, 871; Downing, et al. Science. 1996, 273, 1185; Maciel, et al. Appl. Phys. Lett. 2000, 76, 1978; Shigemura, et al. J. Appl. Phys. 1999, 85, 3413). Que et al. (Que, et al. Journal of Crystal Growth. 2006, 288, 75-78) fabricated Nd3+-doped GeO2—SiO2 thin films by a sol-gel thin coating process and studied them for photonic applications. Rajni and coworkers (Rajni, et al. Journal of The Electrochemical Society. 2005, 152, G456-G459) fabricated an inorganic 20GeO2:80SiO2 thin film also by a sol-gel thin coating process, choosing this ratio due to its lack of clustering in the films.
Monoliths are continuous beds of porous material and can be chemically bonded to the inner walls of a capillary (Hayes & Malik, Anal. Chem. 2000, 72, 4090-4099). Compared to organic, continuous polymeric monoliths, silica-based monoliths prepared by the sol-gel techniques are mechanically stronger and have superior durability in the presence of solvents (Wu, et al. J. Chromatgr. A. 2008, 1184, 369-392). The advantages of monoliths include the ability to control its morphology and the fritless design when used as a column. The size of through-pores, mesopores and skeleton can be controlled by the amount or type of monomer used and by the ratio of monomer, porogen and catalyst (Kato, et al., J. Chromatgr. A. 2002, 961, 45-51).
Monoliths can be chemically modified after fabrication to obtain the desired stationary phase, which can be accomplished in a monomeric or polymeric procedure. A monomeric procedure can be carried out through a direct reaction between silica and the reagent with the desired functional moiety. It can also be accomplished through the introduction of a spacer followed by the reaction of the ligand that introduces the desired functional group. In the case of a polymeric procedure, either a spacer or an anchor is bonded to the silica before the chemical modification is performed (Núñez, et al. J. Chromatogr. A. 2008, 1191, 231-2).
These modifications allow for the development of monolithic columns suitable for different separation modes in HPLC, including ion-exchange, reversed phase (RP), hydrophilic interaction chromatography (HILIC), chiral separations and mixed modes (Núñez, et al. J. Chromatogr. A. 2008, 1191, 231-2). Dulay et al. (Dulay, et al., J. Sep. Sci. 2002, 25, 3) chemically modified sol-gel monoliths by silanizing the sol-gel surface with organochlorosilane or organoalkoxysilane coupling reagents. In 2004, a coating procedure for the modification of a silica monolith by forming a monolithic column with zirconia surface was introduced. Compared to conventional silica monoliths, the zirconia surface increased the stability of the monolith and even facilitated the separation of basic compounds (Shi, et al. Talanta. 2004, 63, 593). Shi et al. (Shi, et al. J. Non-Cryst. Solids. 2006, 352, 4003-4007) developed a novel urea-formaldehyde template for the synthesis of porous inorganic oxide monoliths where silica, zirconia and titania monoliths were successfully prepared. The hydrophilic nature of the template made it possible for the elimination of organic precursors for the preparation of these monoliths (Shi, et al. J. Non-Cryst. Solids. 2006, 352, 4003-4007). Svec et al. have contributed to the fabrication of microchannel monolithic stationary phases in microfluidic devices for chromatographic separation and sample SPE (Yu, et al. Electrophoresis. 2000, 21, 120; Rohr, et al. Electrophoresis. 2001, 22, 3959; Yu, et al. Anal. Chem. 2001, 73, 5088; Yu, et al. J. Polym. Sci. Part A-Polym. Chem. 2002, 40, 755).
While monolithic columns are simple to fabricate and have been successfully employed in capillary electrochromatography (CEC) and high-performance liquid chromatography (HPLC), the existing technology suffers from several limitations. For instances, monolithic columns often crack, shrink, have structural imperfections (e.g., presence of granular voids near the capillary walls), cannot withstand high pressures, and are difficult to seal. Drying produces a pressure gradient in the liquid phase of a gel, leading to a differential shrinkage of the network (Brinker & Scherer, Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, Academic Press: San Diego, Calif., 1990, pp 444-99). Cracking is often attributed to a pore size distribution on a gel. When larger pores are emptied upon evaporation of solvent, the wall adjoining pores is subjected to an uneven stress that can cause cracking (Brinker & Scherer, Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing, Academic Press: San Diego, Calif., 1990, pp 444-99). Accordingly, improved chromatographic and extraction columns are needed.